Method of making printed circuit board

ABSTRACT

A blind hole extending from upper or surface wiring of a printed circuit board to inner or lower wiring, and having an opening larger than the bottom, is formed on a substrate, and a conductor pattern is formed on the bottom and the internal wall of the blind hole to connect the inner wiring with the surface wiring.

The application is a continuation division of application Ser. No.08/164,124, filed Dec. 7, 1993 now abandoned.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to a printed circuit board in which the upperwiring is connected with the lower wiring through the conductor patternon the bottom and internal wall of a blind hole, and a process for themanufacture thereof.

2. Description of the Prior Art

In the manufacture of printed circuit boards, attempts to reduce theland to as little as possible have been made in order to increase theirwiring density. A typical example is the landless substrate, in whichwiring density is increased by eliminating the land when connectingwiring using plated through hole (PTH).

Such a landless substrate has, as FIG. 3 shows for example, a PTH with aplated layer 5 on the internal wall of a through hole 2 formed in thesubstrate 1, to connect the connection line 3 on the top with theconnection line 4 on the bottom. Such a landless substrate is formed byapplying a resin layer in the internal wall of the through hole, platingthe resin layer with solder, and etching off the solder plating, leavinga positive photoresist on the vertical wall (internal wall) of thesolder plating.

However, in such a landless substrate it is impossible to connect theinner lines 6, as shown in FIG. 3, with the vertical wall of the solderplating. On the other hand, it is possible to connect the inner lines 6with the vertical wall of the solder plating without forming a resinlayer on the internal wall of the through hole. FIG. 4 shows theconnection thus formed. (A) is a top view and (B) is a sectional view.As these diagrams show, when the inner lines 6 are thus connected with aplated layer 7, the connection area is only the width×thickness of theinner lines 6. Therefore, the connection of the inner lines 6 with theplated layer 7 lacks reliability.

As FIG. 5 shows, landless via connection is also feasible by forming adrill hole to the middle of the thickness direction of the substrate 1so as to cut the inner lines 6, and forming a plated layer 8 by etchingusing the positive photoresist on the internal wall of the drill hole asdescribed above. In this case, however, connection reliability cannot beobtained.

On the other hand, a structure to connect upper wiring, outer layerwiring, for example, with inner layer wiring, using a blind hole whichreaches the inner wiring, i.e., lower layer wiring, has been known. Theprocess for manufacturing such a structure is shown in FIG. 6. As FIG. 6shows, a photosensitive resin layer 13 is applied to the substrate 11 soas to cover wiring 12 which will become the inner wiring formed on thesubstrate 11 (FIG. 6(a)), the photosensitive layer 13 is irradiatedthrough a mask pattern 14 having a mask 14a the size of the blind holeso as to expose parts other than the mask pattern (FIG. 6(b)), and isdeveloped to form a blind hole 15 extending to the inner wiring 12 (FIG.6(c)). Then, a conductor layer 16 is formed on the bottom and theinternal wall of the blind hole 15 and the surface of the substrate 11(FIG. 6(d)), a dry film photoresist 17 is applied so as to cover theblind hole 15, and the photoresist 17 is exposed through a mask pattern18 having a mask 18a of a size slightly larger than the opening of theblind hole 15 covering the bottom, even if a certain misalignment isconsidered, to expose parts of the photoresist 17 other than the mask18a (FIG. 6(e)). Then, a resist pattern to cover the opening of theblind hole 15 is formed by developing (FIG. 6(f)). Then, a conductorpattern 16a having a land around the opening of the blind hole 15 andbeing connected to the inner wiring 12 are created by etching. Astructure to connect the inner wiring 12 with surface wiring through theconductor pattern 16a is formed by forming the surface wiring (notshown) on the substrate 11 continuous to the land of the conductorpattern 16a using a known method.

Although reliability of connection with the inner wiring is obtained bythe structure as shown in FIG. 6, the requirement of increasing wiringdensity cannot be met because of the presence of a land around theopening of the blind hole.

Therefore, an object of this invention is to provide a printed circuitboard to connect the upper layer wiring with lower layer wiring, and toachieve a high wiring density, as well as a process for manufacturingsuch a printed circuit board.

Another object of this invention is to provide a printed circuit boardin which the inner wiring are reliably connected and to realize a highwiring density.

SUMMARY OF THE INVENTION

To achieve the above objects, this invention relates to a printedcircuit board having upper and lower wiring, wherein a blind hole isformed which extends to said lower wiring and the opening of which islarger than the bottom thereof. A conductor pattern is then formed onthe bottom and the internal wall of said blind hole for connecting saidupper wiring with said lower wiring. It also relates to a process forproducing a printed circuit board comprising the steps of forming ablind hole on a printed circuit board having inner layer wiring, whichblind hole extends to the inner layer wiring and the opening of theblind hole being larger than the bottom thereof, forming a conductorlayer at least around the blind hole and on the bottom and the internalwall of the blind hole, the conductor layer connected with the innerlayer wiring, forming a positive photoresist layer on the surface of theconductor layer, and forming a resist pattern on the bottom and theinternal wall of the blind hole by exposing the pattern through a lightshield pattern larger than the bottom of the blind hole but smaller thanthe opening of the blind hole. Thus developing the pattern, as well asforming a conductor pattern by etching part of the conductor layer otherthan the part of the resist pattern.

In the printed circuit board of the structure as described above,connection with lower wiring can be achieved reliably by the conductorpattern on the bottom of the blind hole, while connection with upperwiring can be achieved reliably by the conductor pattern on the internalsurface of the blind hole, and since no land exists around the openingof the blind hole, high-density wiring is realized.

In the manufacturing method described above, by setting up a positivephotoresist layer on the surface of the electrically conductive layer,in tandem with exposing and developing it through a masking patternpossessing a masking section larger than the bottom of said blind hole,but smaller than the opening of the blind hole, both the bottom and theopening of the blind hole, are covered, yet it is possible to form aresist pattern, with the perimeter of the opening having been completelyremoved. In such exposure, although a part of the photoresist on theinternal wall of the blind hole is irradiated, only little resist on theinternal wall is removed because this internal wall is slanted againstthe direction of irradiation, and the irradiation dose is less than thataround the opening, therefore, by subsequent etching, a conductorpattern continuously extending almost all area of the bottom and theinternal wall of the blind hole.

Other features and advantages of this invention will become apparentfrom the following detailed description of the presently preferredembodiment of the invention, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a printed circuit board accordingto an embodiment of this invention.

FIG. 2 is a process diagram showing an example of the process formanufacturing a printed circuit board according to this invention.

FIG. 3 is a schematic diagram showing an example of a printed circuitboard according to the prior art.

FIG. 4 is schematic diagrams illustrating the problems in the prior art.

FIG. 5 is a schematic diagram illustrating the problems in the priorart.

FIG. 6 is a process diagram showing an example of the process formanufacturing a printed circuit board according to the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention will be described with reference to a preferredembodiment. FIG. 1 schematically illustrates the configuration of aprinted circuit board according to an embodiment of this invention, FIG.1 (A) is a partially cutaway perspective illustration and FIG. 1 (B) isa sectional view thereof. As these drawings show, on a substrate 21 isformed a blind hole 23 extending to an inner wiring 22 (lower wiring),the diameter of which gradually increases from the bottom towards theopening, and on the bottom and the internal wall of the blind hole isformed a conductor pattern 25 connecting the inner wiring 22 with theupper wiring, for example, the top wiring 24. Here, the conductorpattern 25 other than the connection to the top wiring 24 is within theblind hole 23. Therefore, no land exists around the opening of the blindhole 23, which is advantageous to make wiring density higher.

An example of manufacturing such a printed circuit board will bedescribed with reference to FIG. 2. As FIG. 2 shows, an insulativephotosensitive resin layer 21b is formed to cover the inner wiring 22which is a lower wiring consisting of, for example, a copper platedlayer formed on the substrate 21a (FIG. 2(a)). The photosensitive resinlayer 21b is irradiated through the masking pattern 26 having a mask 26athe size of the blind hole to expose the resin layer 21b outside themasked area (FIG. 2(b)), and developed to form the blind hole 23extending to the inner wiring 22 on the substrate 21 (FIG. 2(c)). Here,the blind hole 23 has a shape, the diameter of which increases graduallyfrom the bottom towards the opening. Next, a conductor layer 25' isformed consisting of, for example, a copper plated layer on the bottomand the internal wall of the blind hole 23 and the surface of thesubstrate 21 (FIG. 2(d)). A positive photoresist layer 27 is then formedon the conductor layer 25' using a means such as electro-deposition. Thesystem is then irradiated through a masking pattern 28 having a mask 28aof a size slightly larger than the bottom of the blind hole 23 butsmaller than the opening of the blind hole 23 so as to cover the bottomeven if a certain misalignment is taken into consideration (FIG. 2(e)).At this time, although the photoresist layer 27 on the internal wall ofthe blind hole is also irradiated, the light energy supplied to thephotoresist is less than that applied to other areas, because this areais slanted by, for example, 60-70 degrees against the light irradiatingdirection. By subsequent developing, therefore, a resist pattern 27a isformed on the bottom of the blind hole 23 as well as the internal wall,but the resist around the opening is completely removed (FIG. 2(f)). Byfurther etching, a conductor pattern 25 connected to the inner wiring 22is formed in the bottom and the internal wall of the blind hole 23, butno land is formed around the opening of the blind hole 23. Thesubsequent steps follow known methods, that is, the upper wiring such asthe top wiring (not shown) connected to this conductor pattern 25 isformed by applying a photoresist layer again on the surface of thesubstrate 21, exposing the photoresist layer through a masking pattern,developing, and etching again. By this, the printed circuit board inwhich inner wiring are connected with the surface wiring through theconductor pattern 25 can be obtained (see FIG. 1).

By using such a manufacturing process, the conductor pattern can beformed on the bottom and the internal wall of the blind hole without theaccurate alignment of the masking pattern necessary in conventionalmethods, because the misalignment of the positive photoresist layer andthe masking pattern is compensated by the internal wall of the blindhole.

For example, when the depth of the blind hole is 80 microns, thediameters of its bottom and opening are normally 150 microns and 280microns, respectively. If the conductor layer is etched using a dry-filmphotoresist, as in conventional methods, a land with a diameter of about410 microns is formed around the opening, when the misalignment of thephotoresist layer and the masking pattern is considered. By the methoddescribed above, however, since no land is formed, considerable spacesavings can be achieved.

Advantages over the Prior Art

As described above, since the inner wiring is connected to the lowerwiring through the conductor pattern on the bottom of the blind hole,and is connected to the upper wiring through the conductor pattern onthe internal wall of the blind hole, and the land around the opening ofthe blind hole is eliminated according to this invention, a printedcircuit board with highly reliable connection between upper and lowerwiring, as well as high wiring density, can be provided.

Alternative Embodiments

It will be appreciated that, although specific embodiments of theinvention have been described herein for purposes of illustration,various modifications may be made without departing from the spirit andscope of the invention. In particular, although the embodiment wasdescribed mainly for connection of upper and lower wiring, it will beunderstood by those skilled in the art that this invention can beapplied to multilayer wiring of three or more layers by repetition ofsuch manufacturing steps.

Accordingly, the scope of protection of this invention is limited onlyby the following claims and their equivalents.

We claim:
 1. A process for producing a printed circuit board comprisingthe steps of:forming a blind hole within a printed circuit boardincluding an upper opening, sloped internal wall and a bottom, saidprinted circuit board further having inner layer wiring, said blind holeextending to said inner layer wiring, said upper opening of said blindhole being larger than said bottom thereof; forming a conductor layer atleast around said blind hole, on said bottom of said blind hole, and onsaid sloped internal wall of said blind hole, said conductor layer beingconnected with said inner layer wiring; forming a photoresist layer onthe surface of said conductor layer; positioning a light mask patternlarger than said bottom of said blind hole but smaller than said upperopening of said blind hole in an aligned position over said bottom ofsaid blind hole; forming a photoresist pattern on substantially onlysaid bottom and said sloped internal wall of said blind hole by exposingsaid photoresist pattern through said light mask pattern; and removingthe portion of said conductor layer not covered by said photoresistpattern to define a conductor pattern on substantially only said bottomand said sloped internal wall of said blind hole.
 2. The process ofclaim 1 wherein said photoresist layer is formed on said surface of saidconductor layer using electro-deposition.
 3. The process of claim 2wherein said portion of said conductor pattern is removed using anetching process.
 4. The process of claim 1 further including the step ofproviding wiring on said printed circuit board and connecting saidwiring to said conductor pattern.